Linear actuator, in particular an aircraft brake actuator

ABSTRACT

A linear actuator comprises a piston and a rotary electric motor together with means which are interposed between the electric motor and the piston to define a mechanical system for transforming rotary movement of the motor into linear displacement of said piston, the actuator is characterized in that the electric motor is a vibration type motor comprising at least a stator and a rotor together with excitation means for deforming said stator and/or said rotor in modes of vibration combining tangential vibration and normal vibration for driving said rotor in rotation, and in that the means interposed between the electric motor and the piston include means for mechanically decoupling said piston from said electric motor in the event of the electrical power supply to the elector motor being interrupted.

GENERAL FIELD OF THE INVENTION AND STATE OF THE ART

[0001] The present invention relates to linear actuators.

[0002] Advantageously, but in non-limiting manner, it is applicable tothe field of brakes, in particular aircraft brakes.

[0003] At present, aircraft brakes are actuated by means of hydraulicpistons.

[0004] Such actuators nevertheless present drawbacks associated with theuse of a hydraulic fluid: risks of leakage; need for a power generator;etc. . . .

[0005] For reasons of cost, safety, and limiting pollution, it isdesirable to replace such hydraulic actuators with electric actuators.

[0006] Proposals have already been made for various solutions based onelectromagnetic motors.

[0007] In general, movement from the motor is transmitted to a pistonvia a system of gears and a screw transforming rotary movement intorectilinear movement.

[0008] That mechanism is reversible so that in the event of a failure inthe electrical power supply, the brake is not held on.

[0009] The high speed reduction ratio provided by the gears serves toobtain a high level of force without requiring excessive torque from themotor.

[0010] Such a structure with speed-reducing gears enables the weight andthe cost of the motor to be minimized.

[0011] In spite of that, electromechanical brakes that have been made orproposed still present weight that is too great.

[0012] In addition, the high level of speed reduction in associationwith the inertia of the motor rotor gives rise to very high levels ofenergy consumption when control signals are applied at high frequency(while the actuator is approaching and during anti-lock servo-control)and the level of consumption bears little relationship to the amount ofenergy actually necessary for clamping the disks together.

[0013] So-called “vibration ” (or “ultrasonic ”) electric motorstructures are also known, in particular from patent applications FR97/10948, FR 98/10391, and FR 00/03084 in the name of the Applicantwhich present the advantages of enabling high levels of torque to bedelivered and, for equivalent power, of presenting particularly lowweight.

[0014] Nevertheless, vibration motors suffer from the drawback ofjamming if there is a loss of power, and that is, a priori, difficult tomake compatible with their use in braking systems, in particular foraircraft brakes, since it is usually a requirement in that type ofapplication to be able to release the brakes fully, merely byinterrupting their power supply, specifically for the purpose ofavoiding highly asymmetrical braking while landing.

[0015] Japanese patent application 2001-086 700 discloses a linearactuator which includes a vibration motor and an electromagnet.

[0016] The structure proposed in that document presents the drawback ofpresenting driving strokes that are particularly small. Its weight aninertia remain large.

SUMMARY OF THE INVENTION

[0017] The invention proposes an actuator structure, in particular for abrake actuator, which is of the electrical type, but which makes itpossible to achieve particularly low levels of weight and energyconsumption.

[0018] In particular, the invention provides an actuator structure whichuses a vibration type motor instead of an electromagnetic motor, thusmaking it possible:

[0019] firstly to deliver very high torque at low speed, andconsequently to omit the gearing (saving in weight) and to reduce theapparent inertia (saving in energy consumption); and

[0020] secondly to obtain a significant saving in weight.

[0021] The proposed structure enables the actuator to be releasedcompletely in the event of the electrical power supply beinginterrupted.

[0022] It presents the advantage of enabling a long driving stroke to beobtained, in particular a stroke that is suitable for compensating brakedisk wear.

[0023] The invention also proposes an actuator structure which issuitable not only for performing the main braking function, but is alsosuitable for performing a “parking” brake function.

[0024] Thus, the invention provides a linear actuator comprising a bodyhaving a piston and a rotary electric motor mounted thereon, togetherwith means which are interposed between the electric motor and thepiston and which define a mechanical system that transforms the rotarymovements of the motor into linear displacements of said piston, theelectric motor being a vibration type motor comprising at least a statorand a rotor together with excitation means for deforming said statorand/or said rotor in modes of vibration that combine tangentialvibration with normal vibration so as to drive the rotor in rotation,the means interposed between the electric motor and the piston includingmeans which mechanically decouple said piston from said electric motorwhen the electrical power supply thereto is interrupted, said meanscomprising an electromagnet suitable for holding a complementary polepiece in a given position when it is powered, the actuator beingcharacterized in that the piston is secured to a skirt suitable forsliding on the body in order to guide the piston in its axialdisplacement, the electromagnet and/or the pole piece acting on a partwhich becomes blocked by friction against the guide skirt which issecured to the piston, or on the contrary becomes disengaged, dependingon whether or not the electromagnet is powered.

[0025] Such an actuator is advantageously associated with the variouscharacteristics below, taken singly or in any feasible combination:

[0026] the part which becomes blocked by friction against the guideskirt secured to the piston is a part that is generally bell-shaped,presenting firstly a foot-forming portion secured to one or other of thetwo parts constituting the electromagnet and the pole piece, andsecondly a dome-forming portion which is secured to the other one ofsaid two parts and which constitutes a spring that, when theelectromagnet is powered and the pole piece is pressed thereagainst,pushes back an intermediate portion between the dome-forming portion andthe foot-forming portion so as to block said intermediate portion andthe bell-forming part as a whole by friction against the guide skirtsecured to the piston;

[0027] the electromagnet is suitable for being displaced axially towardsthe piston by an element presenting a thread that meshes with the threadof a nut-forming part, which is itself driven in rotation by thevibration motor, actuation of the vibration motor causing the part whichmeshes with the nut-forming part and the electromagnet to be displacedaxially;

[0028] the element presenting a thread which meshes with the thread ofthe nut-forming part to drive the electromagnet in axial displacement isa hollow element in which there is received a spring that is compressedwith some minimum level of prestress between an end wall presented bysaid hollow element and a part which is suitable for sliding in saidhollow element and for coming into abutment against the electromagnet,the actuator including means for acting, where appropriate, to cause thevibration motor to exert a force that is greater than the prestress ofthe spring;

[0029] the electromagnet, the pole piece, and the element which mesheswith the nut-forming part have a rod passing axially therethrough whichis terminated by a bearing head suitable for coming into abutmentagainst the pole piece, said rod having an outside thread whichco-operates with a complementary thread presented by the nut-formingpart, a second rotary motor being suitable for rotating said rod andthus driving it in axial displacement by virtue of its threadco-operating with the complementary thread presented by the nut-formingpart;

[0030] the guide skirt and the intermediate portion of the bell-formingpart which is designed to become blocked by friction against said guideskirt are coated in a pair of friction layers presenting static anddynamic coefficients of friction which are greater than 0.6 for stressbetween the skirt and the piston of less than 100 megapascals (MPa),said pair of friction layers further presenting surface states suchthat:

[0031] their roughness radii of curvature R satisfy:

[0032] 0.005 millimeters (mm)<<R <<1 mm; and

[0033] the arithmetic mean wavelength λ_(a) and the root mean square(rms) wavelength λ_(q) of the roughness satisfy:

[0034] 0.5 micrometers (μm)<λ_(a) and λ_(a)<10 μm;

[0035] the dome-forming portion forms an angle of 0.50° or less relativeto a plane perpendicular to the axis of the piston;

[0036] the foot-forming portion of the dome-forming part is connected tothe intermediate portion that forms a friction zone via a portion thatlies substantially in the vicinity of the friction cone;

[0037] the actuator includes a ring surrounding the guide skirt in theimmediate vicinity of its zone that is designed to be engaged by thebell-forming part; and

[0038] the actuator includes spring-forming means which hold said ringin the axial direction relative to the guide skirt.

[0039] The invention also provides a brake, in particular an aircraftbrake, comprising friction disks and an actuator as defined above. dr

BRIEF DESCRIPTION OF THE FIGURES

[0040] Embodiments of the invention are described below by way ofexample and with reference to the accompanying drawings, in which:

[0041]FIG. 1 is a section view showing a possible embodiment of theinvention;

[0042]FIG. 2 is a perspective view showing the same embodiment of theinvention;

[0043]FIG. 3 is a diagram of the bell-forming part of the actuator ofFIGS. 1 and 2, with the electromagnet and the pole piece associatedtherewith;

[0044]FIG. 4 is another diagram of the bell-forming part of the actuatorof FIGS. 1 and 2, with the electromagnet and the pole piece associatedtherewith; and

[0045]FIGS. 5a and 5 b are diagrams showing a detail of a possibleadditional embodiment of the invention, the guide skirt, theelectromagnet, the pole piece, and bell-forming part being shown inthese figures both for the case when the electromagnet is not powered(FIG. 5a), and for the case when the electromagnet is powered (FIG. 5b).

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0046] Structure of the Actuator

[0047] The actuator which is shown in the figures comprises a main body1 of generally cylindrical shape and a piston 2 which is designed to bemoved axially relative to said body 1, being guided in axialdisplacement relative thereto by a cylindrical skirt 3. This guide skirt3 is suitable for sliding on said body 1 and it is terminated by thepiston 2.

[0048] For example, when the actuator is used as an aircraft brakeactuator, the piston 2 is intended firstly to be moved in order to takeup any clearance relative to a set of friction disks with which saidactuator is associated, and secondly to apply a force on said diskstending to press them against one another in order to achieve braking.

[0049] The actuator which is shown in the figures further comprises avibration motor 4 which is mounted coaxially inside the body 1.

[0050] The motor 4 is, in particular, advantageously of the typedescribed in the Applicant's French patent applications Nos. FR97/10948, FR 98/10391, and FR 00/03084. It comprises in particular atleast one rotor disk disposed between two stator disks, together withactive elements for tangential and normal deformation which are excitedin such a manner as to deform the stator disks in application ofoperating sequences which serve to drive the rotor disk(s) in rotation.

[0051] These active elements are advantageously of the piezoelectrictype.

[0052] The actuator also includes a nut 5 which extends through therotor-and-stator-forming disks of said motor 4, coaxially therewith.

[0053] The nut 5 presents an inside thread which co-operates with anoutside thread on a coaxial element 6 of cylindrical shape, co-operationbetween said nut 5 and said coaxial element 6 enabling rotary movementof the rotor of the motor 4 to be transformed into movement intranslation along the general axis of the actuator.

[0054] For this purpose, the nut 5 is constrained to rotate with therotor disk(s) of the motor 4 and is held axially relative to the body 1of the actuator by means of abutments 7 which allow it to rotate insidethe body 1 and which oppose reaction forces that are colinear with theaxis.

[0055] The cylindrically-shaped element 6 is a hollow element presentingan end wall 6 a and a cylindrical body 6 b.

[0056] It receives a spring 8 which is compressed between the end wall 6a and a part 9 which terminates the hollow element 6 at its end remotefrom said end wall 6 a.

[0057] The part 9 is suitable for being slid into the hollow element 6when sufficient force for overcoming the spring force is exerted on saidpart 9.

[0058] In the absence of such a force, this part is pushed into aposition where it is blocked by abutments presented by the element 6 andwhere it is constrained to move together therewith both in translationand in rotation.

[0059] Furthermore, the assembly constituted by the nut 5, the element6, the spring 8, and the part 9, has an inner rod 10 passingtherethrough, which rod is terminated by a T-shaped bearing head 10 a.

[0060] A pole piece 13 is slidably mounted on said rod 10 and issuitable for coming into abutment against the bearing head 10 a. Thispole piece 13 co-operates with an electromagnet 12 also mounted to sliderelative to the rod 10.

[0061] A bell-shaped part 11 is mounted between the electromagnet 12 andthe pole piece 13.

[0062] The bell-shaped part 11 presents:

[0063] an annular portion 11 a for being pushed against the inside wallof the cylindrical skirt 3 and which constitutes a zone that is intendedto block the cylindrical skirt 3 and the piston relative to said part 11by friction;

[0064] a portion 11 b constituting the foot of said bell-shaped part 11and which is extended by the portion 11 a that constitutes the frictionzone, the portion 11 b being secured to the electromagnet 12 which issuitable for sliding relative to the body 1; and

[0065] a dome-forming portion 11 c which extends the annular frictionportion 11 a from its end remote from the foot-forming portion 11 b, thedome-forming portion 11 c being a deformable portion constituting aspring which, depending on its position relative to the foot-formingportion 11 b, is suitable either for exerting a radial elastic force onthe friction portion 11 a, this force then blocking the skirt 3 and thepiston 2 relative to the part 11, or else for releasing said portion 11a so as to disengage it from the inside wall of the skirt 3.

[0066] The dome-forming portion 11 c is secured to the pole piece 13,e.g. by means of a screw.

[0067] When the electromagnet 12 is powered, it is suitable for holdingthe pole piece pressed against the electromagnet, with the dome-formingportion 11 c being urged to block the skirt 3 and the piston 2 relativeto the assembly constituted by the electromagnet 12 and the pole piece13.

[0068] In addition, the rod 10 passes through the end wall 6 a of thethreaded element 6 via an orifice that said end wall 6 a presents, thisorifice defining a sleeve having an inside thread that co-operates witha complementary outside thread presented by the rod 10.

[0069] Said rod 10 is also itself suitable for being driven in rotationabout its axis by a motor 14 which drives it via fluting.

[0070] This motor is preferably a vibration motor of the type describedin the above-specified applications.

[0071] The figure also shows a power supply cable C for deliveringelectricity to the motor 4, to the electromagnet 12, and to the motor14.

[0072] Operation

[0073] The above-described structure operates as follows.

[0074] Preparatory Stage

[0075] At rest, the device is as shown in FIG. 1.

[0076] In order to prepare the actuator, the main motor 4 is actuated soas to move the element 6 towards the rear end of the actuator, i.e.towards the motor 14 in the example shown in the figures.

[0077] In its axial displacement, this element 6 begins by entrainingthe rod 10, the pole piece 13, and the part 11, thereby also entrainingthe electromagnet 12.

[0078] Once the electromagnet 12 comes into abutment against the endwall of the housing in which it is received in the body 1, the polepiece 13 approaches the electromagnet 12 such that the dome-formingportion 11 b deforms and the annular friction portion 11 a becomesblocked against the inside wall of the guide skirt 3.

[0079] The electromagnet 12 is then powered, thereby blocking the polepiece 13 and the electromagnet 12 against each other and ensuring thatthe part 11 is blocked relative to the guide skirt 3 and the piston 2.

[0080] The motor 4 is then actuated in the reverse direction, therebydisengaging the bearing head 10 a of the rod 10 from the pole piece 13and the electromagnet 12.

[0081] The actuator is then ready for use: because of the blockingachieved by the angular portion 11 a, all movements transmitted by theelement 6 to the electromagnet 12 are transmitted to the piston 2.

[0082] Main Braking

[0083] Specifically for the purpose of performing main braking, themotor 4 is actuated. Once the element 6 has taken up its clearance andbears against the electromagnet 12, it moves the piston axially until itpresses against the friction disks of the brake; the motor 4 is thenactuated to exert torque which presses hard against the disks andperforms braking.

[0084] The thrust of the electromagnet 12 against the element 6 isconserved even when said element 6 reverses, and this continues so longas the brake disks are under pressure.

[0085] It should be observed that in the event of an untimely loss ofelectrical power supply during braking, the electromagnet 12 and thepole piece 13 are released relative to each other.

[0086] The part 11 is then no longer under tension and the piston 2 isno longer blocked relative to the electromagnet 12 and the element 6.

[0087] Consequently, the proposed structure enables the braking systemto be released immediately in the event of the electrical power supplybeing interrupted.

[0088] Parking Brake

[0089] In order to perform the parking brake function, theabove-described main braking sequence is performed.

[0090] However, the motor 4 is controlled so as to apply clamping forceto the brakes that corresponds to a force greater than that for mainbraking.

[0091] This force overcomes the prestress of the spring 8 located insidethe element 6, such that the part 9 enters into the element 6 andcompresses the spring a little more, thus ensuring in the event of thedisks shrinking due to cooling the spring can expand so as to maintainthe pressure force on the brake.

[0092] The motor 14 is then actuated to reverse the bearing head 10 auntil it comes into contact with the pole piece 13.

[0093] The power to the electromagnet 12 can then be switched offwithout the piston decoupling from said electromagnet 12.

[0094] In this way, the clamping of the brake achieved by the actuatoris maintained after the electrical power supply has been switched off.

[0095] The spring 8 serves to maintain constant clamping force even inthe event of the friction disks of the brake shrinking as they cooldown.

[0096] In order to take off the parking brake, it suffices subsequentlyto apply the reverse sequence: power the electromagnet 12, disengage thehead 10 a from the pole piece 13 by actuating the motor 14, and thenswitch off the power supply to the electromagnet 12.

[0097] Additional Implementation Details

[0098] There follow details of additional features that areadvantageous.

[0099] Coefficient of Friction between the Piston and the Electromagnet

[0100] In order to work with small forces, thereby enabling the weightof the parts to be kept down and also enabling electromagnetic powerconsumption to be kept down, it is preferable for the contact materialbetween the skirt 3 and the part 11 to have a coefficient of frictionthat is as high as possible.

[0101] Such materials are advantageously of the type described in theApplicant's French patent application No. FR 01/00524.

[0102] The contact layers on the skirt 3 and the part 11 areadvantageously constituted by pairs of friction layers presenting staticand dynamic coefficients of friction which are greater than 0.06 forstress between the skirt and the piston of less than 100 MPa, said pairof friction layers further presenting surface states such that:

[0103] their roughness radius of curvature R satisfies:

[0104] 0.005 mm<<R<<1 mm; and

[0105] the arithmetic mean wavelength λ_(a) and the rms wavelength λ_(q)of the roughness satisfy:

[0106] 0.5 μm<λ_(a) and λ_(a)<10 μm

[0107] For examples of materials that satisfy these conditions,reference can advantageously be made to the above-mentioned patentapplication.

[0108] Design of the Portion 11 c Forming a Dome/spring

[0109] The expansion force needed for operation of the dome-formingportion 11 c and for tensioning the annular portion 11 a is given by themaximum load on the piston and by the coefficient of friction. In orderto ensure that this does lead to the electromagnet needing to maintain alarge force, the system is designed so that the angle of inclination α(FIG. 3) between the dome-forming portion 11 c and a plane perpendicularto the axis of the actuator is as small as possible, but withoutbecoming negative since that would prevent the device from releasing inthe event of a power failure.

[0110] By way of example the following value is taken:

[0111] F_(max)=20,000 Newtons (N) on the piston If K is the coefficientof friction, then the force developed by the electromagnet must be equalto: $F_{e} = {\frac{F_{\max}}{K} \times \sin \quad \alpha}$

[0112] which for K=0.5 and α=0.050°, gives F_(e)=400 N.

[0113] Design of the Part 11

[0114] The shape of the part 11, and more particularly of the portion 11d connecting the foot-forming portion 11 b to the friction annularportion 11 a is selected to encourage partial jamming or chocking tominimize the force that needs to be exerted by the diaphragm.

[0115] As shown in FIG. 4, the closer this portion 11 d (segment AB)comes to the normal, the greater the jamming effect; when the segment ABis on the edge of the friction cone, the force to be exerted by theelectromagnet becomes zero, but the device might remain jammed at theslightest variation in the friction coefficient.

[0116] An inclination is therefore selected for the zone 11 d relativeto the normal to the piston so as to be as close as possible to thefriction cone while always remaining outside it whatever the variation.

[0117] For example, for a nominal friction cone corresponding to ahalf-angle of 45°, and a maximum friction cone corresponding to ahalf-angle of 55°, it is advantageous to select an orientation for theportion 11 d so that it corresponds to an angle relative to the normalof 60°, i.e. 55°+5°, where 5° is an additional safety margin.

[0118] In the above example (holding force of 400 N), this makes itpossible to reduce the force that the electromagnet needs to develop toa range of 40 N to 120 N.

[0119] Piston Reinforcing Ring

[0120] The part 11 which bears against the piston may deform it becauseof the high levels of force applied thereto.

[0121] That is why it can be necessary to place a ring 15 outside thepiston for sliding in register with its bearing zone on said part 11. Asshown in FIGS. 5a and 5 b, this ring 15 is advantageously held in placeaxially between two springs 16 of low stiffness which exert axial forceson said ring.

[0122] Such a disposition makes it possible to avoid the ringinterfering with displacements of the piston.

[0123] When the part 11 expands inside the guide skirt 3, the skirtdeforms and presses against the inside of the ring 15 (FIG. 5b).

[0124] However, when the electromagnet 13 is released, the skirt 3slides within the ring 15.

1. A linear actuator comprising a body having a piston and a rotaryelectric motor mounted thereon, together with means which are interposedbetween the electric motor and the piston and which define a mechanicalsystem that transforms the rotary movements of the motor into lineardisplacements of said piston, the electric motor being a vibration typemotor comprising at least a stator and a rotor together with excitationmeans for deforming said stator and/or said rotor in modes of vibrationthat combine tangential vibration with normal vibration so as to drivethe rotor in rotation, the means interposed between the electric motorand the piston including means which mechanically decouple said pistonfrom said electric motor when the electrical power supply thereto isinterrupted, said means comprising an electromagnet suitable for holdinga complementary pole piece in a given position when it is powered, theactuator being characterized in that the piston is secured to a skirtsuitable for sliding on the body in order to guide the piston in itsaxial displacement, the electromagnet and/or the pole piece acting on apart which becomes blocked by friction against the guide skirt which issecured to the piston, or on the contrary becomes disengaged, dependingon whether or not the electromagnet is powered.
 2. A linear actuatoraccording to claim 1, characterized in that the part which becomesblocked by friction against the guide skirt secured to the piston is apart that is generally bell-shaped, presenting firstly a foot-formingportion secured to one or other of the two parts constituting theelectromagnet and the pole piece, and secondly a dome-forming portionwhich is secured to the other one of said two parts and whichconstitutes a spring that, when the electromagnet is powered and thepole piece is pressed thereagainst pushes back an intermediate portionbetween the dome-forming portion and the foot-forming portion so as toblock said intermediate portion and the bell-forming part as a whole byfriction against the guide skirt secured to the piston.
 3. An actuatoraccording to claim 2, characterized in that the electromagnet issuitable for being displaced axially towards the piston by an elementpresenting a thread that meshes with the thread of a nut-forming part,which is itself driven in rotation by the vibration motor, actuation ofthe vibration motor causing the part which meshes with the nut-formingpart and the electromagnet to be displaced axially.
 4. An actuatoraccording to claim 3, characterized in that the element presenting athread which meshes with the thread of the nut-forming part to drive theelectromagnet in axial displacement is a hollow element in which thereis received a spring that is compressed with some minimum level ofprestress between an end wall presented by said hollow element and apart which is suitable for sliding in said hollow element and for cominginto abutment against the electromagnet, the actuator including??? meansfor acting, where appropriate, to cause the vibration motor to exert aforce that is greater than the prestress of the spring.
 5. An actuatoraccording to claim 3 or claim 4, characterized in that theelectromagnet, the pole piece, and the element which meshes with thenut-forming part have a rod passing axially therethrough which isterminated by a bearing head suitable for coming into abutment againstthe pole piece, said rod having an outside thread which co-operates witha complementary thread presented by the nut-forming part, a secondrotary motor being suitable for rotating said rod and thus driving it inaxial displacement by virtue of its thread co-operating with thecomplementary thread presented by the nut-forming part.
 6. An actuatoraccording to any one of claims 2 to 5, characterized in that the guideskirt and the intermediate portion of the bell-forming part which isdesigned to become blocked by friction against said guide skirt arecoated in a pair of friction layers presenting static and dynamiccoefficients of friction which are greater than 0.6 for stress betweenthe skirt and the piston of less than 100 MPa, said pair of frictionlayers further presenting surface states such that: their roughnessradii of curvature R satisfy: 0.005 mm<<R<<1 mm; and the arithmetic meanwavelength λ_(a) and the rms wavelength λ_(q) of the roughness satisfy:0.5 μm<λ_(a) and λ_(a)<10 μm.
 7. An actuator according to any one ofclaims 2 to 6, characterized in that when the electromagnet is stuck,the dome-forming portion forms an angle of 0.5° or less relative to aplane perpendicular to the axis of the piston.
 8. An actuator accordingto any one of claims 2 to 7, characterized in that the foot-formingportion of the dome-forming part is connected to the intermediateportion that forms a friction zone via a portion that lies substantiallyin the vicinity of the friction cone.
 9. An actuator according to anyone of claims 2 to 8, characterized in that it includes a ringsurrounding the guide skirt in the immediate vicinity of its zone thatis designed to be engaged by the bell-forming part.
 10. An actuatoraccording to claim 9, characterized in that it includes spring-formingmeans which hold said ring in the axial direction relative to the guideskirt.
 11. A brake, in particular an aircraft brake, characterized inthat it includes friction disks and an actuator according to anypreceding claim.